1. Field of the Invention
The present invention relates generally to image forming devices and particularly to media pick mechanism. More specifically, the present invention discloses a media sensing apparatus to detect a leading edge and a trailing edge of a media sheet at the pick point of the media sheet.
2. Description of the Related Art
Most image forming devices have a media picking mechanism that separates and feeds media sheets from a media stack in a media input tray into a main media path. The media stack and the picking device continually move relative to each other so as to keep the media picking mechanism in contact with a topmost media sheet. There are many types of media picking mechanisms, most of which rely upon certain assumptions regarding the general characteristics of friction between the mechanical components of the auto compensator mechanism and the media sheet. If the design assumptions are met, then only a single top most media sheet is separated from the media stack and fed into the system. However, if these assumptions are not satisfied, certain pick and feed errors can result.
The most common pick and feed problems are:
1) Fail to feed errors (FF), where pick tires slip on the media sheet, and the media sheet either fails to move, or does not move far enough to be fed into the main media path;
2) Double or multi-feed errors (DF), where more than one media sheet is fed because subsequent media sheets stick together;
3) Shingle feed errors, where more than one sheet is fed but due to some overlap, the sheets are shingled but appear as one piece of sheet.
While the media picking mechanism can be designed to minimize the frequency at which the above mentioned errors occur under nominal operating conditions, the mechanism will always be susceptible to these types of errors due to a large range of variables. Variables such as media type, media weight, media texture, customer loading conditions, environmental effects, wear of the mechanism and other unexpected variations can affect the reliability of the mechanism, and therefore, some feed issues are inevitable. While there are many design approaches used to address these problems (auto compensating arms, dual friction separators, corner bucklers, etc.), it would be cost prohibitive to design a mechanism that could handle every combination of such a wide range of variables. Common designs work around a nominal center point and allow for as much tolerance as possible to address these variations. This mandates that the design should have a method to deal with the outlying conditions that the design is not intended to accommodate. In addition to these variables, there could be an unexpected scenario when a user may load media that is either shorter or longer than expected.
Generally, image forming apparatus have sensors at stationary locations in the media path to monitor the movement of the media sheets and are located downstream from a pick point of the media picking mechanism. In the event, one of the pre-described errors occur, the image forming apparatus is configured to detect an unexpected transition at one or more of these sensors, and the image forming apparatus is then forced to stop the media sheet in the media path and post a “media jam” because the image forming apparatus cannot accurately predict the condition of the media sheet. While the common jam scenarios mentioned above do not create a condition where the media sheet is obstructed, the image forming apparatus does not have a means to detect this and therefore stops the media sheet. These “obstructed” jam scenarios require the media sheet to be cleared by the user. During the jam removal, there is opportunity for the user to damage the media sheet while clearing the media path by grabbing and pulling the media sheet out, possibly ripping the sheet, wrinkling it, or otherwise making it unsuitable for future use. The user may then likely throw the media sheet in the trash, resulting in higher usage cost to them. Further, in order to gain access to the media path, most of the image forming apparatus requires the user to open covers/doors and remove components, such as a print cartridge. This increases the chances that damage can occur to either the machine or the print cartridge, resulting to further increased expense and inconvenience. Users, who are unable to perform these actions, may additionally require a service call and/or warranty action.
To reduce these problems and create a better customer experience, an inexpensive means of detecting a media sheet at the pick point before the media sheet enters the media path is needed so that the machine firmware can make better decisions regarding the movement of the media sheet, thereby avoiding unnecessary jam conditions caused by undesired media sheet behavior.
Given the foregoing, it would be desirable to be able to sense the leading and trailing edges of a media sheet being picked while it is still in the media input tray such that the image forming device firmware can detect a leading edge and a trailing edge of each media sheet at or adjacent to the pick point itself rather than waiting for the media sheet to be fed into the media path and then being sensed. This enables the image forming device to prevent media jam conditions by avoiding scenarios that can occur when the image forming apparatus picks the media sheet and then has to wait to sense the media sheet until it is in the media path.